PROJECT SUMMARY The human auditory brain is remarkably plastic but we still do not know just how plastic it is, particularly when it comes to higher cognitive functions like speech perception and music enjoyment. These are uniquely human skills which are difficult (if not impossible) to study in animal models. Post-lingually hearing impaired cochlear implant (CI) users have provided a very interesting platform to study plasticity in response to auditory stimuli sent to a non-standard peripheral frequency-place function. This is because the frequency-place function imposed by a CI is different for each individual (due to differences in cochlear size, electrode location, and neural survival patterns) and can be quite different from the ?natural?, physiological frequency-place function. This proposal seeks to investigate auditory plasticity in traditional post-lingually hearing impaired CI users (Aims 1 and 2) as well as in a novel population of CI users who have normal hearing in the unimplanted ear (Aims 3 and 4). This new clinical population of CI users with single-sided deafness (CI-SSD) is ideal to study adaptation to frequency-place functions without the confounds that complicate interpretation of results in listeners with partial or complete deafness. In Aim 1 we will study the combined effect of using electrode selection and listener-selected frequency allocation tables (FATs) in postlingually hearing impaired CI users. There are indications that these two interventions (both of which affect the frequency-place function in different ways) may enhance speech perception and sound quality. In Aim 2 we will test the use of a smartphone-based app to facilitate the selection of customized frequency-place functions in the real world rather than in the acoustically controlled environment of a laboratory or a clinic. In Aim 3 we will use three behavioral tests of adaptation to frequency- place functions in CI-SSD patients. Data from one of these tests will allow us to meet an important sub-aim: to assess the validity of existing acoustic models of cochlear implantation, and to create improved acoustic models whose sound quality and speech intelligibility are similar to those obtained with a CI. Lastly, in Aim 4 we will study the effect of user-selected frequency-place functions on bilateral music sound quality and bilateral unmasking, also in CI-SSD patients. The proposed experiments will yield novel and unique information about how postlingually hearing impaired humans adapt to modified frequency-place functions, and thus will make an important contribution to studying basic auditory function in humans. We believe that the proposed research also has important translational implications that are likely to influence clinical practice as well as research with CI users.